This invention relates to a solvent composition which selectively absorbs sulfur dioxide from effluent gases containing carbon dioxide. More particularly, this invention relates to the discovery that an aqueous alkanolaminium carboxylate solution preferentially absorbs sulfur dioxide, has a high absorption capacity for sulfur dioxide, and suppresses the rate at which absorbed sulfur dioxide oxidizes to non-regenerable sulfate salts. Compared to other known sulfur dioxide absorption solutions, solvents of this invention have lower heats of desorption and hence may be thermally stripped of dissolved sulfur dioxide more easily and economically than other known selective sulfur dioxide absorption solvents.
Many important industrial units produce vent gas streams containing sulfur dioxide and carbon dioxide. Sulfur dioxide is a pollutant and must be removed from such gas streams before they may be safely or legally vented to the atmosphere. One prime example of a process which generates a gas stream requiring treatment is the hydrodesulfurization of petroleum stocks. In hydrodesulfurization the sulfur content of petroleum is converted to and removed as hydrogen sulfide. The hydrogen sulfide gas stream is then often treated by the Claus process to recover the sulfur values thereof as elemental sulfur. However, sulfur recovery by the Claus process is not complete and a tail gas is produced which may contain from 3% to 15% of the original sulfur values, predominately as a mixture of hydrogen sulfide and sulfur dioxide, together with considerable quantities of carbon dioxide. In the past, Claus plant tail gases have been incinerated to convert toxic hydrogen sulfide to sulfur dioxide and the gas stream vented to the atmosphere. Another prime source of sulfur dioxide effluent gases is the combustion of sulfur-containing fossel fuels in the production of power, generally electrical power. Combustion of high sulfur fuels produces stack gases containing large volumes of carbon dioxide and objectionable quantities of sulfur dioxide. Such gases must be treated to remove sulfur dioxide before they may be safely vented to the atmosphere.
A number of non-reversible absorption or adsorption processes have been proposed for removing sulfur dioxide from such gas streams prior to venting them to the atmosphere. Many proposed processes react sulfur dioxide with limestone or other inorganic compounds, in solid or solution form, to remove sulfur dioxide as a sulfite or sulfate salt. However, such methods require the use of uneconomically large quantities of reagents in greater than stoichiometric amounts and produce large quantities of waste solids or liquids which themselves require disposal, thus presenting secondary pollution problems.
Processes for the reversible absorption of sulfur dioxide in a regenerable solvent have been suggested. Thus, alkali and alkaline earth metal hydroxides and carbonates, ammonium hydroxide and aqueous ammonia have been tried as regenerable sulfur dioxide solvents. However, such solvents have exhibited poor sulfur dioxide pick-up, low sulfur dioxide absorption capacity, extremely high stream stripping requirements and side stream reactions such as sulfur dioxide disproportionation or oxidation to produce non-regenerable sulfate salts which accumulate within the solvent and diminish its capacity to absorb sulfur dioxide. These and other disadvantages have made the use of such solvents unattractive.
Likewise, amines have been suggested as regenerable sulfur dioxide solvents but have been found unsatisfactory. Amines are reported to absorb sulfur dioxide by, at least in part, an irreversible reaction that produces dithionates which cannot be regenerated by thermal stripping. Further, at stripping temperatures above about 212.degree. F. dissolved sulfur dioxide irreversibly reacts with amines to produce organic sulfur compounds and polymeric products.
Recently, U.S. Pat. No. 3,904,735 has reported that certain trialkanolamines and their alkoxylated and sulfite derivatives, when used under specific processing restrictions, may be employed as selective regenerable solvents for the absorption of sulfur dioxide from gases which also contain carbon dioxide. The utility of such solvents was predicated upon the discovery that triethanolamine which otherwise would be subject to the problems of sulfur dioxide disproportionation and solvent degradation undergoes such adverse side reactions only to a negligible extent when certain critical processing conditions are closely observed. Particularly, the absorption temperature must not exceed 212.degree. F., the stripping temperature must not exceed 266.degree. F., and at least one mole of water for each mole of absorbed sulfur dioxide must be maintained in the solvent. Additionally, triethanolamine was unexpectedly found to be the least active of the possible trialkanolamine solvents in promoting the oxidation of dissolved sulfur dioxide to non-regenerable sulfate salts.